3GPP (The 3rd Generation Partner Project) standardization organization is working out the next generation of wireless communication standard which is named LTE (Long Term Evolution). In a physical layer interface, the new standard adopts OFDM (Orthogonal Frequency Division Multiplexing) technology, which is different from conventional CDMA (Code Division Multiple Access) technology. In LTE, OFDMA (Orthogonal Frequency Division Multiple Access) is used in downlinks (DL) and SC-FDMA (Single Carrier Frequency Division Multiple Access) is used in uplinks (UL). The technology used in the new standard is effective for multi-path propagation, with adoption of frequency domain equalization reducing complexity of the conventional time domain equalization, and is more suitable for bandwidth high-speed data transmission.
In the LTE system, according to scheduling of eNB (evolved NodeB), UE (User Equipment) sends SRS (Sounding Reference Signal) in the uplink. The SRS is designed as a wideband sounding signal to facilitate UL frequency-selective scheduling as well as other purposes such as UL power control, time tracking, etc. In TDD (Time Division Duplexing) system, the SRS may also be used for downlink scheduling via exploitation of channel reciprocity. According to the current standardization of LTE (see Non-Patent Document 1 below), main procedures for SRS transmission include: eNB (or base station) broadcasts to the UEs (or mobile station) common cell-specific SRS configuration parameters in a designated cell as needed; the eNB sends UE-specific RRC signaling (Radio Resource Control signaling) to the UE to configure dedicated SRS parameters for a designated UE; then the UE sends the SRS according to the parameters in the configured bandwidth and subframes periodically. For example, the eNB configures to the UE {2, 5, 10, 20, 40, 80, 160, 320} ms as periodicity (SRS transmission interval). Hereinafter, this periodical SRS transmission is described as “periodic SRS (periodic SRS transmission)”. In addition, in LTE, the eNB may configure one-time periodic SRS transmission to the UE by using the UE-specific RRC signaling and the eNB may semi-statically send the RRC signaling (e.g. per 100 ms).
The 3GPP is also working out LTE-Advanced (Advanced-LTE) which is likely the major enhancements to LTE. The LTE-Advanced will be introduced in Release 10 after a correction and improvement phase in LTE Release 9. The LTE-Advanced shall fulfill the requirements as set by ITU (International Telecommunication Union). In LTE-Advanced, the SRS may be used for CSI (Channel State Information) estimation at multiple cells exploiting the channel reciprocity in addition to the above purposes for LTE. The SRS design needs to take into account the LTE-Advanced features such as multiple transmit antennas, CoMP (Coordinated Multipoint Transmission/Reception), supporting heterogeneous networks, etc.
Regarding the SRS transmission for the LTE-Advanced, it is proposed one scheduled (a periodic/one shot) SRS scheme (see Non-Patent Document 2 below). In this scheme, the SRS parameters for scheduled SRS are configured via higher layer signaling. The scheduled SRS is triggered using SRS-indicator included in an UL grant (an UL transmission permission signal) and the eNB dynamically sends the UL grant (e.g. per 1 ms). Additionally, it is proposed to define a new PDCCH (Physical Downlink Control Channel) format for triggering the scheduled SRS for many UEs at the same time without the need for scheduling the PUSCH (Physical Uplink Shared Channel). The scheduled SRS is a kind of “one-shot” allocation/transmission by nature.
Furthermore, in LTE-Advanced, carrier aggregation is supported in order to support wider transmission bandwidth (see Non-Patent Document 3 below). It is stated that LTE-Advanced should support wider bandwidth than LTE Release 8, up to 100 MHz. Typically contiguous spectrums is used, but the non-contiguous spectrums should be also supported considering reasonable UE complexity. An obvious way of fulfilling this requirement is to use carrier aggregation, where multiple component carriers are aggregated to the desired LTE-Advanced system bandwidth. In principle, the component carriers may be either contiguous or non-contiguous in frequency. The base station (BS) and the user equipment may communicate on the wider transmission bandwidth with aggregate multiple component carriers. For example, the base station and the user equipment may communicate on 100 MHz bandwidth which is aggregated by five 20 MHz component carriers (5×20 MHz=100 MHz). The component carrier is respective frequency bandwidth (or respective carrier frequency) which consists of the wider transmission bandwidth. In details, the base station and the user equipment may communicate by using aggregated multiple DL CCs (Downlink Component Carriers(s)) and aggregated multiple UL CCs (Uplink Component Carrier(s)). Here, the number of the DL CCs and the number of the UL CCs may be the same, i.e., Symmetric carrier aggregation. Alternatively, the number of the DL CCs and the number of the UC CCs may be different, i.e., Asymmetric carrier aggregation.
To well support the carrier aggregation, cross-carrier scheduling (scheduling over the component carriers) and relevant control signaling is widely discussed. Regarding the control signaling of resource assignments for PDSCH (Physical Downlink Shared Channel) and PUSCH, the following mechanisms are supported (see Non-Patent Document 4 below): the PDCCH (for the DL or UL) on a component carrier assigns the PDSCH resources on the same component carrier and the PUSCH resources on a single linked uplink component carrier (UL CC). The LTE (Release 8) PDCCH structure, which includes coding, CCE (Control Channel Element)-based resource mapping and etc, is used on each component carrier. The same formats for the LTE (Release 8) PDCCH (DCI (Downlink Control Information) format) are used on each component carrier. Furthermore, the PDCCH on a component carrier may assign the PDSCH resources or the PUSCH resources in one of multiple component carriers using a carrier indicator (CI) field, where the DCI formats are extended with a fixed 3 bit CI field. The eNB may assign the PDSCH resources or the PUSCH resources to the UE by using the PDCCH including the CI field which indicates the scheduled component carrier.    Non-Patent Document 1: “3GPP TSG RAN E-UTRA Physical layer procedure (Release 8)”, 3GPP TS 36.213 V8.8.0, 2009-09.    Non-Patent Document 2: “Channel sounding enhancements for LTE-Advanced”, 3GPP TSG RAN WG1 Meeting #59, R1-094653, November 2009.    Non-patent Document 3: “Carrier aggregation in LTE-Advanced”, 3GPP TSG RAN WG1 Meeting #53bis, R1-082468, June, 2008.    Non-patent Document 4: “TP for TR36.814 on downlink control signaling for carrier aggregation”, 3GPP TSG RAN WG1 Meeting #59, R1-094959, November 2009.
However, in the above conventional techniques, there is no concrete description on what kind of exchange is made between the eNB and the UE to transmit the scheduled SRS. Namely, it is only worded that the SRS parameters for the scheduled SRS are configured via higher layer signaling, there is no concrete description on how the eNB configures the UE to send the scheduled SRS, and how the UE sends the scheduled SRS to the eNB.
Regarding the SRS transmission in LTE-Advanced, in order to achieve efficient scheduling by the eNB, it requires shorter period and/or wider bandwidth SRS transmission, which in turn results in consumption of more SRS resources. Performance of the SRS transmission may become a limiting bottleneck for efficiently scheduling by the eNB. Additionally, the eNB should consider interferences in the cell caused by sending the SRS from UEs. For example, when a UE sends the uplink data (transport block for UL-SCH (UL-Shared Channel)) by using the PUSCH, if another UE sends the SRS in the same subframe, it comes up with the interference in the cell because the PUSCH resources and the SRS resources are overlapped (collided).
The present invention has been made in view of the foregoing circumstances, and its object is to provide a mobile communication method, system, a base station and a user equipment and integrated circuits used therein, which may perform flexible transmission control for the scheduled SRS transmission, and achieve more efficient transmission control (scheduling) between the eNB and the UE.